Since the discovery of high-temperature superconducting (HTS) materials (superconducting above the liquid nitrogen temperature of 77 K) there have been efforts to research and develop various technology and engineering applications using such HTS materials. In thin film superconductor devices, the most progress has been made with fabrication of Josephson junctions and microwave devices utilizing an oxide superconductor including yttrium, barium, copper and oxide in the well-known basic composition of YBa2Cu3O7-x (hereinafter referred to as Y123). At liquid nitrogen temperatures and in high magnetic fields, the Jc of Y123 is superior to those of the bismuth, thallium and mercury based HTS materials. Thus, Y123 has generally been the preferred material for thin film and bulk applications.
Even though Y123 is the material of choice for HTS applications, it still has a few problems. One problem is that Y123 has one of the lowest Tc's among (RE)Ba2Cu3O7-x materials (hereinafter referred to as RE123) which can limit Jc at the liquid nitrogen temperature (since Jc depends on Tc; Jc≈(1−T/Tc)3/2). Another problem is that compared with SmBa2Cu3O7 (Sm123) and NdBa2Cu3O7 (Nd123) films, Y123 films have a much rougher surface morphology which is detrimental for any device application and imposes a materials challenge. Still another problem is that it has been known that Nd123 has a larger Jc in high magnetic fields than Y123. Hence, it has been important to continue development of RE123 films for various HTS applications. Previously, there have been some efforts to fabricate high quality RE123 films. Sm123 and Nd123 films with good superconducting properties (Tc>90 K and Jc>106 Amperes per square centimeter (A/cm2)) and smooth surface morphology have been fabricated using Pulsed Laser Deposition (PLD), Molecular Beam Epitaxy (MBE), and Coevaporation techniques. However, the optimization of film properties has only been achieved by using barium-rich targets, post-annealing of the films, or changing the stoichiometry of the targets, and it has been difficult to reproducibly fabricate high quality films.
There have been several attempts to overcome these difficulties by using a buffer layer to produce RE123 films. For example, U.S. Pat. No. 5,232,900 describes the use of a non-superconducting buffer layer. U.S. Pat. No. 5,512,541 describes the deposition of superconducting films on a superconducting single crystal substrate. U.S. Pat. No. 5,629,268 describes the deposition of multilayers on a bottom superconductor without damaging its superconducting properties. Kwon et al., Appl. Phys. Left., 62, 1289-1291 (1993), describe a buffer layer of yttrium-doped Pr123 in order to improve the superconducting properties of ultra-thin Y123 films.
Despite these earlier efforts, the need to find additional buffer materials suitable for reproducible deposition of various RE123 films has remained. Thus, an object of the present invention is buffer materials suitable for reproducible deposition of various RE123 films.
Another object of the present invention is to simplify the process conditions needed to form various RE123 films.